1. Field of the Invention
The present invention relates to a radiation curable resin composition which has excellent characteristics for adhering to substrates, characteristics avoiding damage when scratched by a nail (hereinafter referred to as "scratch resistance"), light resistance, heat resistance, high temperature marring resistance, fingerprint attachment resistance, and printing aptitude, and, more particularly, to a radiation curable resin composition suitable for use as a printable heat-resistant protective coat which is used for information recording media such as thermosensible recording cards and thermosensible photographic printing paper.
2. Description of Related Art
A protective coating layer is provided in order to improve heat resistance and resistance to marring on the surface of recording media such as rewritable-type thermosensible recording cards and thermosensible photographic printing paper. A UV-curable type acrylate resin composition conventionally used for surface protection of plastic boards and as a protective coat for optical disks has been also used as the material for forming such a protective coating layer. Japanese Patent Application Laid-open No. 4-149280 proposes an overcoating composition for optical disks which comprises tripropylene glycol diacrylate, 2-(2-ethoxyethoxy)ethyl acrylate, and a photopolymerization initiator. The overcoating composition disclosed here exhibits low irritation to skin, has a low viscosity, and when used as an overcoating for optical disks, possesses sufficient coated film hardness and superior resistance to marring. Although such an overcoating material excels in mar resistance at normal temperatures, the material has poor shock resistance when subjected to thermal head contact when used as a protective coat for thermosensible record cards. Thus, mar resistance at high temperatures is a problem of this composition which must be solved. In addition, the protective coating surface produced by this overcoating composition shows inferior resistance to fingerprint attachment. When touched by a finger, a fingerprint trace is easily attached, which not only results in impaired surface transparency, but also makes it difficult to identify the character information and the like recorded in a lower layer of the protective coat layer.
A multi-layer coating of thermosetting resin consisting of a combination of a specific primer material containing an unsaturated double bond and an organopolysiloxane hard coating was proposed in Japanese Patent Application Laid-open No. 4-106161. The articles coated by this material are characterized in their possession of excellent mar resistance, abrasion resistance, and surface glossiness, as well as superb weather resistance of their adhesion characteristics. The organopolysiloxane hard coat disclosed here forms a coating film containing siloxane bonds when hardened. It is expected that the hard coat will exhibit superior mar resistance at high temperatures. This patent further proposes an addition of fine particles of an inorganic oxide to surface coating films in order to improve the surface hardness of the coating film. However, because the hardening is performed by heat, the addition of fine particles of inorganic oxide not only lengthens the hardening time and thereby decreases the productivity, but also a thermosensible layer is caused to deteriorate if used as the thermosensible recording media. Thus, the material is not practical. Moreover, although the overcoat layer with a polysiloxane structure formed possesses superior mar resistance and heat resistance, the coating film has only poor flexibility. If used on a rewriting type thermosensible record card or on thermosensible record paper, cracks are easily produced. In addition, the surface of the overcoating produced in this manner easily repels printing ink and makes favorable printing difficult.
Japanese Patent Application Laid-Open No. 103328/1995 discloses a radiation curable coating composition comprising, 100 parts by weight (hereinafter simply referred to as parts) of polymerizable compounds consisting of:
(A) 20 to 80 parts of an acrylated bisphenol-A epoxy resin wherein the resin comprises 1 to 3 bisphenol-A moieties in average,
(B) 5 to 50 parts of a compound comprising a acrylated dipentaerythritol derivative that may comprise polymerised caprolacton and hydrogenated acrylic acid groups,
(C) 0 to 40 parts of a compound comprising two acrylate groups and a 1,3-dioxane moiety, and
(D) 0 to 40 parts of monomers or prepolymers which are copolymerizable with the compounds (A), (B), and (C);
0-10 parts of a photopolymerization initiator; and
0-150 parts of an organic solvent.
The inventors of the invention of this laid-open patent application claim that this composition has a low viscosity, is easy to apply, and is hardened in a short period of time by irradiation using ultraviolet rays or electronic beams, has excellent processability such as superior adhesion to a substrate and superb bending characteristics, and produces products with a favorable gloss. However, the coating film produced from this composition, when used as a heat resistant protective coating, has inferior scratch resistance and fingerprint attachment resistance.
An object of the present invention is to provide a radiation curable resin composition which is suitable for producing a printable heat resistant protective coating used for thermosensible type information recording media such as a thermosensible recording card and a thermosensible printing photographic paper.